Various printing systems include a moving belt that carries one or more sheets of print media through a predetermined path while images are formed on the media sheets. An example of such a device is an inkjet printer that includes a moving belt. The moving belt carries one or more media sheets past two or more marking stations that eject ink drops onto the media sheets. The marking stations are located at different positions along the path of the belt. In some embodiments, each marking station is configured to eject ink having a single color. Each marking station forms a portion of a color image using one ink color on each media sheet, and the juxtaposition of the different colored inks from the marking stations forms a full-color image on the media sheets. One common example of such a printing system forms images using a combination of inks having cyan, magenta, yellow, and black (CMYK) colors. In one such system, each marking station includes one or more printheads that are configured to eject ink drops onto the media sheets at predetermined locations to form ink images on the media sheets.
One aspect of printing systems that carry print media on a moving belt is the stability of each media sheet as the media sheet passes each printhead for the ejection of ink onto the media sheet. Different printer embodiments incorporate various components that generate a force to hold each media sheet flat against the moving belt. Some printers incorporate a vacuum source that is coupled to vacuum platen. The vacuum platen includes a plurality of passageways or ports to enable air to be drawn through the platen towards the vacuum source. The vacuum platen is positioned and oriented so it is adjacent the back side of the belt as the belt carries the print media by the marking stations. The belt may include a plurality of holes to enable the vacuum source to exert pressure on the media sheets through the belt. Thus, the air being pulled through the platen urges the media against the belt to help stabilize the media while it is being printed. Other embodiments may include an electrostatic member positioned adjacent to the back side of the belt that generates an electrical charge opposite an electrical charge on the media sheets, attracting the media sheets and the moving belt to the electrostatic member. Still other embodiments may include mechanical members, such as gripper bars or hold-down rolls that push the media sheets against the front-side of the moving belt, and consequently push the moving belt against a support member, such as a backer roll, positioned on the back side of the moving belt.
As the belt travels through the print zone, friction between the belt and other structural members of the printer in the print zone generates drag on the belt. The drag causes the belt to stretch as the belt moves through the print zone. In addition to the friction between the belt and structures in the print zone, the additional force applied to each media sheet to hold the media sheet against the belt in the print zone produces friction and drag between the moving belt and the structural members in the print zone. The total drag force exerted on the belt changes as each sheet enters the print zone, passes through the print zone, and exits the print zone. Since one or more media sheets may pass through the print zone at different times, the total frictional force and drag on the belt may vary, with the force increasing when more media sheets are in the print zone and decreasing when fewer media sheets are in the print zone.
The changes in frictional forces also change the total drag and stretch of the belt as the belt passes the marking stations. The changes in the stretch of the belt, in turn, result in changes to the relative position of the media sheets and the marking stations as the media sheets pass the marking stations. The changes in relative position may cause errors in the formation of images on each media sheet, referred to as registration errors. Thus, improvements that reduce or eliminate registration errors in printing systems that carry one or more media sheets on a belt are beneficial.